The group-III nitrides, for example GaN, are promising wide band-gap semiconductors for optical devices in the blue and ultraviolet (S. Nakamura, 1997a, 1998b), high-temperature and high-power device applications (H. Morkos et al., 1994; T. P. Chow et al., 1994, J. C. Zolper et al., 1996).
However, the reliability of current state-of the art GaN-based devices such as blue emitter and high-temperature devices is limited. Blue diode laser lifetimes are short. This is widely attributed to the fact that most GaN devices are grown on lattice mismatched substrates such as sapphire which results in high dislocation densities, typically about 1010 cmxe2x88x922.
Epitaxial GaN films have recently attracted much interest based on their optoelectronic applications as blue-ultraviolet optoelectronic devices and high temperature transistors (S. Nakamura, 1997, 1998). Since bulk GaN substrates are not available currently, the films are generally grown on sapphire, SiC, GaAs, or Si substrates. These substrates provides poor lattice and thermal expansion matching to GaN which lead to very high densities of structural defects. The identification of an appropriate matched substrate for epitaxial growth might enable the preparation of high quality devices with these semiconductor materials.
One growth process for preparing single crystal films of GaN relies on the vapor phase reaction between GaCl3 and NH3 in a hot-walled reactor, such as a halide vapor phase epitaxy (HVPE) system. Sapphire substrates are often used because they are readily available. However, since sapphire is not lattice matched to GaN, GaN and sapphire have very different thermal expansion coefficients. Accordingly, the resulting GaN has poor crystalline quality, having high dislocation densities and other lattice imperfections. Even so, growth of GaN on sapphire is still common. Furthermore, attempts have been made to reduce the occurrence of these dislocations and other lattice imperfections by providing buffer layers, such as AlN or ZnO, between the sapphire and the GaN. However, the defects from the substrate mismatch propagate through the buffer layers to the GaN film.
Though the first demonstration of the fabrication of single crystal GaN occurred 30 years ago, interest in these materials for real-world optoelectronic devices has grown only in the last 5-6 years as material quality has improved and controllable p-type doping has finally been achieved.
A primary difficulty in producing high quality GaN single crystal has been the lack of lattice matching substrates such that high quality GaN single crystal epitaxial films could not be produced. Since high quality bulk GaN substrates have not been available, GaN films are generally grown on sapphire, SiC, or Si substrates. However, III-V nitride compounds having the wurtzite structure which is hexagonal in symmetry, in general, have much smaller lattice constants (a-axis dimension=3.104 xc3x85 for AlN, 3.180 xc3x85 for GaN and 3.533 for InN) as compared to the currently available semiconductor substrates which typically have cubic symmetry. Accordingly, sapphire, SiC, and Si provide poor lattice, as well as thermal, matching to GaN which can lead to very high densities of structural defects.
The first blue GaN-based light emitting diodes (LEDs) and lasers, are now commercially available. They are fabricated from epitaxial GaN grown on sapphire substrates (S. Nakamura, 1997). The best published lifetime for a GaN-based laser on sapphire is only tens of hours, probably due to the high density of crystal defects. Recently a laser lifetime of 10,000 hours has been reported for devices fabricated on lateral overgrowth epitaxial material on patterned sapphire substrate (S. Nakamura, 1997a, 1998b, 1998c).
Apparently the GaN that laterally overgrows on the SiO2 mask (in between the mask openings) has dislocation densities that are orders of magnitude lower than material grown directly on sapphire. Lasers fabricated in this low defect density material have much longer life time.
This epitaxially laterally overgrowth (ELOG) technique involves the growth of a GaN buffer layer on a substrate of, for example, Si, GaAs, Sapphire or SiC. A pattern of SiO2, for example, stripes, is then grown on the GaN buffer layer. The SiO2 are about 0.2 xcexcm in thickness and preferably covers about two-thirds of the buffer layer. An example may have 6-8 xcexcm wide SiO2 stripes with 4 xcexcm spacing. As the growth of GaN is continued, the GaN does not grow on the SiO2 stripes but, rather, only in the grooves. As the GaN growth in the grooves reaches the height of the SiO2 stripes, the GaN continues to grow up, but also begins to grow laterally from the sides of the GaN ridges to eventually form one continuous film. The defect density of the ELOG GaN film can be on the order of 107/cm3, with a reduced number of threading dislocations in GaN layer compared with GaN grown directly on the substrate without the SiO2 stripes. The original substrate material can then be removed, for example via etching, but the SiO2 grooves are still trapped inside the GaN material. Furthermore, removal of the original substrate can damage the GaN material.
Recently in GaN multi-quantum-well-structure laser diodes (LDs) grown on GaN substrates were demonstrated (S. Nakamura, et al., 1998). The LDs showed a lifetime longer than 780 h despite a large threshold current density. In contrast, the LDs grown on a sapphire substrate exhibited a high thermal resistance and a short lifetime of 200 h under room-temperature continuous-wave operation.
Because of high dissociation pressure of nitrogen over GaN ( greater than 70 kbar at 2300xc2x0 C.), no one has succeeded in making large bulk GaN single crystal substrates. Currently, bulk crystals with dimensions of only a few millimeters can be obtained with high pressure synthesis (20 kbar and 1600xc2x0 C.) (I. Grzegory et al., 1993) and by hydride vapor phase epitaxy (HVPE) on SiC or sapphire substrates with subsequent substrate removal by reactive ion etching, laser pulses, or by polishing. Accordingly, GaN is usually made by heteroepitaxy onto lattice mismatched substrates such as sapphire (S. Nakamura, 1997a) and silicon carbide (Yu V. Melnik et al., 1997) with subsequent substrate removal by reactive ion etching or wet chemical etching, by laser pulses or by polishing. Each of these removal procedures can cause residual strain, changes in chemical composition of epitaxial films, etc. In addition, not only are the lattice constant of the GaN film and substrate very different, but so are the thermal expansion coefficients, creating additional inducement for the creation of dislocations. These two factors, lattice constant mismatch and very different thermal expansion coefficients, can result in GaN epitaxial films with high densities of dislocations (1010 cm2), regions of built-in strain, and cracks which often occur due to thermal stress during cooling.
This suggests that if low dislocation density bulk GaN substrates were available, device life times approaching the 50,000 target for reliable CD-ROM storage devices could readily be achieved. Similar improvements can be expected with respect to the reliability of other GaN-based devices such as heterojunction bipolar transistors and modulation-doped field effect transistors for high-temperature electronics and uncooled avionics.
The subject invention pertains to a method and device for producing large area single crystalline III-V nitride compound semiconductor substrates with a composition AlxInyGa1-x-y N (where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, and 0xe2x89xa6x+yxe2x89xa61). In a specific embodiment, a crystal GaN substrates with low dislocation densities (xcx9c107 cmxe2x88x922) can be produced. These substrates, for example, can be used to fabricate lasers and transistors. Large bulk single crystals of III-V compounds can be produced in accordance with the subject invention by, for example, utilizing the rapid growth rates afforded by hydride vapor phase epitaxy (HVPE) and growing on oxide substrates such as lithium gallate LiGaO2 substrates. Lithium gallate has a close lattice mismatch (xcx9c1%) to GaN. LiGaO2 has an orthorhombic structure with lattice parameters of a=5.402 xc3x85, b=6.372 xc3x85 and c=5.007 xc3x85. Bulk single crystals of LiGaO2 can be grown from a melt by the Czochralski technique. Lithium gallate crystals were obtained from Crystal Photonics, Inc. and subsequently sliced and polished on both sides. A thin MOVPE (metal organic vapor phase epitaxy) GaN film can be grown on the lithium gallate substrates to protect the oxide substrate from attack by HCl during HVPE. The oxide substrate can be self-separated from the GaN film after special substrate treatment procedure and cooling process. Examples of oxide substrates include LiGaO2, LiAlO2, MgAlScO4, Al2MgO4, and LiNdO2. In a specific embodiment, AlN can be grown on LiAlO2, preferably after surface nitridation.
The subject invention also relates to an apparatus which can alternately perform MOVPE and HVPE, without removing the substrate. This eliminates the need to cool the substrate between the performance of the different growth techniques. The subject invention can utilize a technique for the deposition of GaN which can alternate between MOVPE and HVPE, combining the advantages of both. In this process, during HVPE, trimethylgallium (TMG) can first be reacted with HCl in the source zone of the hot wall reactor (see FIG. 1A) to form chlorinated gallium species. For example, TMG and HCl can be reacted according to the following reaction:
Ga(CH3)3+HClxe2x86x92GaCl+3CH4
Preferably, the methyl radicals can be converted to methane gas such that negligible carbon is incorporated in the GaN films. The stream can then be combined with NH3 in the downstream mixing zone and directed toward a substrate where deposition of GaN occurs. For example, the stream can be combined with NH3 resulting in GaN deposition in accordance with the following reaction:
GaCl+NH3xe2x86x92GaN+HCl+H2
The advantages of this technique include: the ability to deposit GaN by either MOVPE or HVPE in the same reactor, high growth rates, rapid reactant switching, lower background impurities with HCl (the Cl retains metal impurities in the vapor phase), in-situ etching, elimination of HVPE source problems and finally improvement of NH3 cracking.
Preferably, LiGaO2 substrate nitridation is utilized for GaN film/LiGaO2 substrate self-separation which can cause the GaN film to xe2x80x9clift offxe2x80x9d the substrate, such that substrate removal in HCl by wet chemical etching is not needed.
Changes in the surface morphology, chemical composition and crystal structure of the (001) LiGaO2 substrate as a function of nitridation agent, temperature and time, and showed the influence of surface morphology of the nitrided layer on the subsequent growth of GaN films and film/substrate self-separating.
The subject invention relates to a method for producing III-V nitride compound semiconductor substrates, comprising the steps of: growing a first III-V nitride compound semiconductor onto an oxide substrate by MOVPE; and growing an additional III-V nitride compound semiconductor by HVPE onto the first III-V nitride compound semiconductor grown by MOVPE. This method can be utilized to grow a first and additional III-V nitride compound semiconductors each having a composition given by AlxInyGa1-x-y N (where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, and 0xe2x89xa6x+yxe2x89xa61). The first and the additional III-V nitride compound semiconductors can have different compositions or each have the same composition. The oxide substrate can have an orthorhombic structure with a good lattice match to the first III-V nitride compound semiconductor. For example, the oxide substrate can be selected from the group consisting of LiGaO2, LiAlO2, MgAlScO4, Al2MgO4, and LiNdO2. Preferably, the oxide substrate has a surface area for III-V nitride compound semiconductor growth of at least 10xc3x9710 mm2.
In a preferred embodiment, the first and the additional III-V nitride compound semiconductors are both GaN. At least 0.2 xcexcm of GaN can be grown during the MOVPE growth step. The step of growing GaN onto the LiGaO2 substrate by MOVPE can be conducted in a low pressure horizontal cold-wall MOCVD reactor with triethylgallium (TEGa) and ammonia (NH3) as precursors and N2 as a carrier gas.
Advantageously, the oxide substrate, for example the LiGaO2 substrate, can be maintained at an elevated temperature between the step of growing GaN by MOVPE and the step of growing GaN by HVPE. Also, the step of growing GaN by MOVPE and the step of growing GaN by HVPE can each take place in the same reactor.
The MOVPE grown GaN can serve to protect the LiGaO2 substrate from attack by HCl during the HVPE growth of GaN. If desired, additional GaN can be grown by MOVPE onto the GaN grown by HVPE, producing a high quality surface. After the final layer is grown, the GaN can be cooled in for example nitrogen flow, to room temperature. The step of growing additional GaN by HVPE can involve the step of first reacting trimethylgallium (TMG) with HCl in a source zone of a hot wall reactor to form a stream comprising a chlorinated gallium species. For example, the TMG can be reacted with HCl according to the following reaction:
Ga(CH3)3+HClxe2x86x92GaCl+3CH4.
Preferably, the methyl radicals are converted to methane gas such that neglible carbon is incorporated in the GaN. The step of growing additional GaN by HVPE can further involve the step of combining the stream with NH3 in a downstream mixing zone and directing the stream toward the GaN grown by MOVPE on the substrate where growth of additional GaN can occurs. Upon combining the stream with NH3 the deposition of GaN can occur. For example, the stream can be combined with NH3 resulting in GaN deposition in accordance with the following reaction:
GaCl+NH3xe2x86x92GaN+HCl+H2
After the step of growing additional GaN by HVPE the LiGaO2 can be removed from the GaN by, for example, wet chemical etching. Preferably, the GaN can be lifted off the LiGaO2 substrate. In a preferred embodiment, prior to the growth of GaN onto a LiGaO2 substrate by MOVPE, nitridation of the LiGaO2 substrate can be performed. This substrate nitridation can cause a reconstruction of the substrate surface and the formation of a thin layer of nitrided material having the same orientation as the substrate. This substrate nitridation can involve the steps of: heating the substrate in the presence of nitrogen; and exposing a surface of the substrate to NH3. Preferably the substrate is heated for a period of time ranging from about 10 minutes to 15 minutes in a temperature range of about 800xc2x0 C. to about 850xc2x0 C., and the substrate surface is exposed to NH3 for a period of time ranging from about 30 seconds to about 10 minutes in a temperature range of about 800xc2x0 C. to about 900xc2x0 C.
After growing additional GaN by HVPE, the LiGaO2 substrate and the GaN can be separated. This separation can be accomplished by the application of mechanical force such that the GaN lifts off of the LiGaO2 substrate. After the GaN is separated from the LiGaO2 substrate, the LiGaO2 can then be reused to grow additional GaN.
The subject method can be used to produce a large area free standing GaN crystal, having a dislocation density less than 108 cmxe2x88x922. The surface area of these crystals can be at least 10xe2x88x924 m2, and have been as large as a 2 inch diameter circular wafer. In a specific embodiment, a GaN crystal has been produced with a dislocation density less than 107 cmxe2x88x922 and a useable substrate area greater than 10xe2x88x922 m2.
The subject invention also relates to a method of preparing the surface of an oxide substrate, comprising the steps of: heating an oxide substrate in the presence of nitrogen; exposing a surface of the oxide substrate to NH3. This method is applicable to oxide substrates such as LiGaO2, LiAlO2, MgAlScO4, Al2MgO4, and LiNdO2. The oxide substrate, for example LiGaO2, can be heated for a period of time ranging from about 10 minutes to about 12 minutes in a temperature range of about 800xc2x0 C. to about 850xc2x0 C. The surface of the oxide substrate can be exposed to NH3 for a period of time ranging from about 30 seconds to about 10 minutes in a temperature range of about 800xc2x0 C. to about 900xc2x0 C. The substrate can be heated in the presence of nitrogen, for example flowing N2 over the oxide surface at a flow rate in the range from about 2 L/min to about 5 L/min. This method can improve the smoothness of the surface of the oxide substrate.
The subject invention also pertains to a device for producing GaN crystals having a means for performing metal organic vapor phase epitaxy (MOVPE) on the surface of the substrate and a means for performing hydride vapor phase epitaxy (HVPE) on a surface of a substrate. The device can transition from MOVPE to HVPE in situ.
Advantageously, the substrate does not have to be removed from the device between MOVPE and HVPE and, therefore, the substrate can be maintained at elevated temperatures during transition from MOVPE to HVPE.